A wireless LAN in compliance with IEEE 802.11 standard, for example, has a problem in that, when a server or the like on a wired LAN communicates with a mobile wireless terminal over the wireless LAN, the frequency band to be used by a wireless base station or access point (AP) is narrower than that of the wired LAN. In a wired LAN in compliance with 100 Base-T, the maximum transmission rate is 100 Mbps, and the maximum effective transmission rate is about 80 to 90 Mbps. In contrast, in a wireless LAN employing the OFDM scheme in the 5.2 GHz band in compliance with IEEE 802.11a, the maximum transmission rate is 54 Mbps, and the maximum effective transmission rate is about 20 Mbps. In a wireless LAN employing the spread spectrum scheme of 2.4 GHz band in compliance with IEEE 802.11b, which is most widely used, the maximum transmission rate is 11 Mbps, and the maximum effective transfer rate is about 5 Mbps.
It is known that, in order to address the problem above, an area covered by the base station is divided into a plurality of respective small sectors with a plurality of respective directive antennas of the base station. As long as the sectors are completely separated from one another and are not subject to interference with one another, the separate communications in the respective sectors can be provided concurrently, and the throughput of each terminal in each sector is improved. In the mobile communication system, the frequencies allocated to the respective sectors are typically adjusted between the adjacent sectors to reduce interference between the adjacent sectors. In the wireless LAN, however, the number of channels available without causing interference is limited, and hence it is not desirable to allocate a plurality of channels to one base station.
Kawabata et al. in Japanese Patent Publication JP-A-2000-59287 laid-open on Feb. 25, 2000, for example, disclose a mobile wireless communication system which includes wireless base stations and terminal stations provided with respective directive antennas. The base station has a directivity determining pattern for setting the directivity of its antenna in accordance with timings. Neighboring base stations using the same frequency use different directivity determining patterns. Remote base stations repeatedly use the same directivity determining patterns. The directivity determining patterns are determined by associating the directivities of the antennas with timings which have been derived by equally dividing the frames of communication channels by the number of directivity determining patterns.
Toshimitsu et al. in Japanese Patent Publication JP-A-2001-309424 laid-open on Nov. 2, 2001, for example, disclose a wireless base station which transfers signals in the form of time-division multiplexed frames to and from a plurality of wireless terminals. The wireless base station includes: a beam forming section which simultaneously forms a plurality of spatially divided beams, a plurality of antenna elements for radiating the beams toward the terminals to transfer signals to and from the terminals, and a scheduling processing section for allocating communication bands to the respective wireless terminals for a plurality of frames corresponding to at least one of the beams in a manner to avoid mutual interference between the signals to be transferred in different frames.
Yano et al., Communication Society Conference of The Institute of Electronics, Information and Communication Engineers 2002, B-8-186 to 189, pp.483-486, proposes integral management of a plurality of base stations having respective directive beams, so that the throughput is improved while avoiding interference between the base stations. In this case, however, the protocol of the wireless LAN is specialized, and hence existing terminals can not access the base stations.
The inventor has recognized that the throughput per wireless terminal can be improved, by dividing an area covered by a wireless base station into a plurality of sectors, causing the base station using respective directive antennas to communicate with the wireless terminals in the respective sectors, and allocating communications to the adjacent sectors in a time divisional manner.
An object of the invention is to increase the throughput per terminal in a wireless LAN.
Another object of the invention is to enable a larger number of wireless terminals over a wireless LAN to communicate with a base station in a narrow frequency band.